Enemy health bar over them and destroy actor

Hi I am having trouble adding the health bar so it appears OVER the enemy, the script I have puts it in the top of the screen. I want it so when I am so far away its either not visible or it is very small above their head. And also when their health reaches 0 the enemy is destroyed. And I have multiple enemies so how can I make it to where all aren't destroyed? Here is my script, and thanks for your help! I know its a lot!

using UnityEngine; using System.Collections;

public class EnemyHealth : MonoBehaviour { public int maxHealth = 100; public int curHealth = 100;

 public float healthBarLength;

 // Use this for initialization
 void Start () {
     healthBarLength = Screen.width / 2;
 }
 
 // Update is called once per frame
 void Update () {
     AddjustCurrentHealth(0);
 }
 
 void OnGUI() {
     GUI.Box(new Rect(10, 40, healthBarLength, 20), curHealth + "/" + maxHealth);    
 }
 
 public void AddjustCurrentHealth(int adj) {
     curHealth += adj;
     
     if(curHealth < 0)
         curHealth = 0;
     
     if(curHealth > maxHealth)
         curHealth = maxHealth;
     
     if(maxHealth < 1)
         maxHealth = 1;
     
     healthBarLength = (Screen.width / 4) * (curHealth / (float)maxHealth);
 }

}